In video standards for standard-definition (SD) and high-definition (HD) video signal transmission, a color video signal is represented by digitized samples of digital luminance, Y, and the color differences, Cb and Cr. The digital luminance and color difference values (Y, Cb, Cr) of each pixel (i.e., the pixel data) can be derived from the three primary analog signals representing RGB, i.e., E′R, E′G, and E′B, according to world-wide video standards set forth in ITU-R BT.601 and ITU-R BT.709. Each of the values (Y, Cb, Cr) in the luminance/color differences color space is a function of all three primary analog values in the R, G, B color space. The pixel luminance data, Y, corresponds to the overall brightness of the pixel in the video signal, and is the weighted sum of the gamma-corrected RGB components of the color video signal.
The video signal is typically received by a display system, such as a television, and displayed on a display device. Typical display devices include projector displays and flat-panel displays. Projector displays, such as digital micro-mirror device displays (DMD), liquid-crystal displays (LCD) and liquid-crystal on silicon (LCOS) displays, utilize a lamp to provide a light source to project images onto a projection surface, such as a screen, while flat-panel displays, such as LCDs, utilize a backlight to display images for direct viewing.
The quality of the displayed images is strongly affected by the pixel luminance data from the input video signal and the brightness of the ambient lighting. For projector displays the image quality is also affected by the reflectivity of the surface onto which the image is projected. To display clear and sharp images, it is necessary to adjust the brightness and contrast of the image to compensate for the variations of the pixel luminance data from the input video signal and various ambient lighting conditions. Typically, with conventional display systems, the viewer can perform such brightness and/or contrast adjustments manually.
Nevertheless, the input video signal generally does not represent a fixed image. Rather, the images usually are changing along with the video content. Thus, oftentimes the optimal brightness and contrast for one scene will be different for another scene, e.g., one scene can include outdoor/daytime footage and the next scene can include indoor/evening footage. Moreover, conditions in the viewing environment can change, e.g., the ambient lighting in the room can change from natural light during the day to artificial light during the evening, thereby affecting the optimal brightness and contrast levels. In order to optimize the perceived image quality, the viewer would be required to adjust constantly the brightness and contrast of the display device.
Accordingly, it is desirable to provide a method and system for automatically adjusting the brightness and contrast of images from a video source displayed by a display system. The method and system should be adaptable to the video content displayed and to the viewing environment, should not require extensive computational resources, and should be efficient.